objective-c - 使用带有块参数的 NSURLConnection 异步 URL 请求

Question

我正在尝试适当地使用 NSURLConnection 的 sendAsynchronousRequest 方法，但我遇到了 block 参数的问题。我在调用上方构建了块，并使用 __block 变量尝试检索其中收集的数据。我已经确认块内的数据是正确的，所以块工作得很好，让块数据的检索成为问题。我在方法末尾的赋值和返回语句没有得到正确的值，我认为这是因为在赋值发生之前块尚未完成执行。如何在不中断主线程的情况下“等待”块完成执行后再执行分配？

- (NSData *) sendAsynchronousURLRequest: (NSMutableURLRequest *)request withResponse: (NSURLResponse *)response andError:(NSError *)error
{
    // Prepare the block handler.
    __block NSURLResponse *retrievedBlockResponse;
    __block NSError *retrievedBlockError;
    __block NSData *retrivedBlockData;
    void(^requestHandler)(NSURLResponse*, NSData*, NSError*) =
    ^(NSURLResponse *blockResponse, NSData *blockData, NSError *blockError)
    {
        retrievedBlockResponse = blockResponse;
        retrivedBlockData = blockData;
        retrievedBlockError = blockError;

        // NOTE: Correct data is here...
        NSLog(@"Data Inside Block:\n%@", [NSMutableDictionary DictionaryFromJSONData:blockData errorReport:nil]);
    };

    // Send the asynchronous request (operationQueue initialized elsewhere in class).
    [NSURLConnection sendAsynchronousRequest:request queue:operationQueue completionHandler:requestHandler];

    // NOTE: Reference to data cannot be established here!
    // Establish references to block variables and return NSData object.
    response = retrievedBlockResponse;
    error = retrievedBlockError;
    return retrivedBlockData;
}